Curing of ink by radiation, and in particular ultraviolet (UV) radiation curing, has become popular. UV curable inks can be cured after printing by the application of UV light. UV curable inks often include monomers that form polymers by free radical polymerization. The growing end of each polymer chain is a radical that reacts with additional monomers, transferring the radical to the end of the chain as each monomer is added. A photoinitiator may be used to form the first radicals to begin the polymerization process. As an example, the photoinitiator is capable of absorbing UV light to generate radicals to react with the monomers.
Two types of photoinitiators can be used in UV curable compositions. Type I photoinitiators are unimolecular photoinitiators that undergo a hemolytic bond cleavage upon absorption of UV light, forming radicals. Type-II photoinitiators are bimolecular photoinitiators. Type-II photoinitiators are a system that includes a photoinitiator with a synergist, which together can form radicals upon exposure to UV light. Some type-II photoinitiators react by hydrogen abstraction from the synergist to the photoinitiator.